1. Field of the Invention
This invention relates to an apparatus for driving a solid state image pickup device used in a color video camera provided with a single image pickup device.
2. Related Background Art
As the light-receiving accumulation modes of a solid state image pickup device used in a video camera, there are known the frame accumulation mode in which signal charges are accumulated in a charge accumulation type photoelectric converting element for one frame period (1/30 sec.) and the field accumulation mode in which signal charges are accumulated in a charge accumulation type photoelectric converting element for one field period (1/60 sec.).
FIG. 1 of the accompanying drawings, is a schematic view of an interline transfer CCD known as a solid state image pickup device, and arrows therein indicate the movement of signal charges from light-receiving portions 1 to vertical transfer portions 2 in the frame accumulation mode.
That is, solid-line arrows indicate the movement of signal charges in odd fields, and broken-line arrows indicate the movement of signal charges in even fields, and further, .phi.1-.phi.4 designate the four-phase driving signal electrodes of a vertical transfer CCD. A horizontal transfer portion 33 is a two-phase CCD and is driven by a horizontal transfer pulse applied to electrodes .phi.5 and .phi.6. The charges transferred by the horizontal transfer portion 33 are put out to the outside through an amplifier 34.
The driving in the frame accumulation mode is shown in the time chart of FIG. 2 of the accompanying drawings.
In FIG. 2, a frame synchronizing signal FLD is at a high level (hereinafter referred to as H level) for a period of time substantially corresponding to the transfer period of charges accumulated in the odd fields and at a low level (hereinafter referred to as L level) for a period of time substantially corresponding to the transfer period of charges accumulated in the even fields. V.BLK is a vertical blanking signal, and the period during which V.BLK is at L level is a vertical blanking period. Further, V1-V4 are electrode voltages corresponding to the transfer electrodes .phi.1-.phi.4.
As is apparent from FIG. 2, in the case of the frame accumulation mode, the voltage Vl of the electrode .phi.1 assumes the highest one of three levels at the timing of times T2, T4, ..., and as indicated by solid-line arrows in FIG. 1, the signal charges of the odd fields are transferred from the light-receiving portions 1 to the vertical transfer portions 2, whereafter they are read out as video signals. On the other hand, the voltage V3 of the electrode .phi.3 assumes the highest one of three levels at the timing of times T1, T3, ..., and as indicated by broken-line arrows in FIG. 1, the signal charges of the even fields are transferred from the light-receiving portions 1 to the vertical transfer portions 2 and are likewise read out as video signals.
Accordingly, in such driving by the frame accumulation mode, the accumulation time of the signal charges of each field is 1/30 sec. However, each of the areas in V1-V4 of FIG. 2 in which two oblique lines intersect each other indicate a period of time during which a predetermined synchronous transfer pulse is generated.
However, in the driving by the frame accumulation mode, when a moving object is photographed, the video signals of the odd fields and the even fields overlap each other by one field and therefore, an unpleasant field afterimage is created.
In contrast, in the driving by the field accumulation mode wherein signal charges are accumulated in one picture element for one field period (1/60 sec.), the signal charges of all picture elements are read out for each field and the picture element signals of two horizontal lines vertically adjacent to each other are compositely read out and therefore, no field afterimage is created. Here, the light-receiving portions 1 are charge accumulation type photoelectric converting elements.
FIG. 3 of the accompanying drawings shows the movement of signal charges from the light-receiving portions 1 to the vertical transfer portions 2 by the field accumulation mode, for example, in an interline transfer CCD, and FIG. 4 of the accompanying drawings shows the timing chart thereof.
First, in the case of the odd fields, at the timing of times T2, T4, ... of FIG. 4, the signal charges of the light-receiving portions 1 of two adjacent horizontal lines which provide the odd fields are added together in the vertical transfer portions as indicated by solid-line arrows in FIG. 3, and in the case of the even fields, at the timing of times T1, T3, ... of FIG. 4, the signal charges of the light-receiving portions 1 of two adjacent horizontal lines which provide the even fields are added together in the vertical transfer portions 2 as indicated by broken-line arrows in FIG. 3 and are read out.
Accordingly, the signal charges of all picture elements are read out for each field and therefore, the accumulation time of the signal charges is 1/60 sec. and no field afterimage is created.
Now, the driving method by the field accumulation mode wherein the signal charges of two horizontal lines are combined together in this manner and video signals are read out, whereby the accumulation time of the signal charges is 1/60 sec. poses no problem even if the signal charges of two horizontal lines are combined together where black-and-white video signals and video color signals are to be obtained by the use of complementary color filters comprising cyan, magenta and yellow, and also permits separation of color information, but is inferior to primary color filters in the S/N ratio of color signals and color reproducibility. Also, stripe-like filters using primary colors cannot sufficiently obtain brightness signals and are inferior in resolution. On the other hand, where color video signals are to be obtained by the use of other primary color filters (for example, a primary color mosaic filter) than stripe-like primary color filters, the presence of the process of combining and reading out the signals of two horizontal lines gives rise to color mixture and such filters cannot be used and after all, only the driving by the frame accumulation mode can be applied, and this has led to the disadvantage that a field afterimage is created. This will hereinafter be described in more detail.
Consider a single-plate color television camera using a single interline transfer CCD having attached thereto other primary color filters than stripe-like primary color filters. Taking as an example a primary color filter of such an arrangement as shown in FIG. 5 of the accompanying drawings, each of light-receiving portions 1 has applied thereto color coatings indicated by R, B and G.
As is apparent from FIG. 5, in the read-out of even fields by the field accumulation mode, three primary colors of R, G and B signals are added together in the vertical transfer portions without any problem and without color mixture, but in the case of the odd fields, R signal and B signal are mixed in the vertical transfer portions and therefore, in this case, it becomes impossible to obtain R and B signals and after all, when use is made of other primary color filters than stripe-like primary color filters, it has been impossible to obtain color video signals in the driving method by the field accumulation mode.